Back to EveryPatent.com
United States Patent |
5,196,856
|
Litchford
,   et al.
|
March 23, 1993
|
Passive SSR system utilizing P3 and P2 pulses for synchronizing
measurements of TOA data
Abstract
The range from which a collision avoidance system at an Own station can
receive SSR interrogations and reply messages from Other stations, is
extended by utilizing P2 pulses for timing TOA measurement in the event
P1-P3 pulse pairs are unavailable from the scanning main beam or its side
lobes. The amplitude of the P2 pulses in the SLS radiation pattern being
greater than the P1-P3 side lobes of the main beam over an angular sector
of about 60.degree. centered on the main beam can insure reception of P2
pulses at much greater ranges than P3 pulses contained in the main beam
side lobes can be reliably received. Using P2 timing, interlaced Mode A
and Mode C reply messages contained in a main beam burst reply sequence
are separated into two "families" of TOAs, the Mode C (altitude) TOAs
always being longer than the Mode A TOAs by 13 .mu.sec. A "true" TOA is
obtained by subtracting an appropriate time period from the TOA of each
family, from which identity, altitude and range information is readily
derived. The system continuously adapts to the best instantaneously
available timing pulses, alternating between P3 timing and P2 timing
throughout the time it takes for a main beam rotation of the received
SSRs, thereby extending the operation area of multiple TOA measurements
from multiple SSRs which, in turn, provides added safety and reduced false
alarms compared to prior passive collision avoidance systems.
Inventors:
|
Litchford; George B. (Northport, NY);
Hulland; Burton L. (Long Beach, NY)
|
Assignee:
|
Litchstreet Co. (Northport, NY)
|
Appl. No.:
|
908183 |
Filed:
|
July 1, 1992 |
Current U.S. Class: |
342/455; 342/32; 342/398 |
Intern'l Class: |
G01S 003/02 |
Field of Search: |
342/455,453,398,32
|
References Cited
U.S. Patent Documents
4733241 | Mar., 1988 | Litchford et al. | 342/453.
|
Primary Examiner: Blum; Theodore M.
Attorney, Agent or Firm: Brumbaugh, Graves, Donohue & Raymond
Claims
We claim:
1. A method for detecting at an Own station the proximity of
transponder-equipped Other stations, comprising the steps of:
(a) receiving side lobe suppression signals (SLS) transmitted by SSR
stations located within operating range of said Own station;
(b) receiving reply messages transmitted by transponder-equipped Other
stations in response to main beam interrogations of Others by said SSR
stations during a predetermined time period following the reception of
each SLS signal;
(c) determining from said received SLS signals and said reply messages a
differential time of arrival (TOA) value for each reply message;
(d) associatively storing said reply messages and TOAs for several of the
most recent reply messages representative of a predetermined number of
interrogation periods;
(e) selecting from the TOA values stored in step (d) those TOA values that
differ by 13 microseconds;
(f) subtracting 19 microseconds from the longer TOA values and 6
microseconds from the shorter TOA values selected in step (e), for
establishing a common TOA value representative of what the TOA value would
have been had main beam interrogation signals, rather than SLS signals,
been used in step (c) to determine TOA values; and
(g) decoding reply messages having longer TOA values as altitude codes and
reply messages having shorter TOA values as identity codes.
2. The method as set forth in claim 1, wherein said method includes the
further steps of:
(h) selecting the largest differential TOA related to each identified Other
station; and
(i) producing a threat alert in response to any such selected differential
TOA below a predetermined value.
3. The method as set forth in claim 1 or claim 2, wherein said method
includes the further steps of:
(j) determining the differential altitude of each said identified Other
station with respect to Own station's altitude;
(k) storing as entries said differential altitude data associatively with
said identity and TOA data; and
(l) discarding all stored associated groups of identity and differential
altitude data with an altitude difference greater than a predetermined
value above Own's altitude or a second predetermined value below Own's
altitude.
4. The method set forth in claim 1, wherein in step (b) said predetermined
time period is initiated by a P2 pulse contained in a received SLS signal,
and wherein the method includes the further steps of:
correlating from the entries of step (d) those TOA values that are
substantially duplicated a predetermined number of times, and
storing separately each such correlated TOA values for a storage period at
least as long as the longest SSR beam rotation period.
5. In a method for detecting at an Own station the proximity of a
transponder-equipped Other station, comprising the steps of:
(a) receiving identity and altitude interrogation signals composed of
differently spaced P1-P3 pulse pairs transmitted by each SSR within
operational range of said Own station when said Own station is within the
main beam or main beam side lobes of an SSR;
(b) receiving only P2 pulses or P1-P2 pulse pairs transmitted by the side
lobe suppression (SLS) control signal pattern associated with the main
beam of each said SSR when said Own station is within operational range of
such SLS P2 pulses or P1-P2 pulse pairs;
(c) receiving reply messages transmitted by transponder-equipped Other
stations in response to main beam interrogation of Others by said SSR
stations during a predetermined period following reception of each P3
interrogation pulse or the P2 pulse of each SLS signal; and
(d) when P1-P3 pulse pairs are not received in step (a) and P2-only pulses
or P1-P2 pulse pairs are received in step (b), determining from the time
relationships between each received P2 pulse and each received reply
message elicited by an associated interrogation signal, the identity and
altitude of each said Other station and differential time of arrival (TOA)
data for each identified Other station with respect to each said SSR.
6. The method set forth in claim 5, wherein step (d) includes the steps of:
(d.sup.1) associatively storing successive reply messages and TOA values
for each for a predetermined number of interrogation periods;
(d.sup.2) selecting from the TOA values stored in step (d.sup.1) those TOA
values that differ by 13 microseconds;
(d.sup.3) subtracting 19 microseconds and 6 microseconds from the longer
and shorter TOA values, respectively, selected in step (d.sup.2) to
establish a common TOA value representative of what the TOA value would
have been had P3 pulses, rather than P2 pulses, been used in step
(d) to determine TOA values; and
wherein said method includes the further steps of:
(e) selecting the largest differential time of arrival related to each said
identified Other station; and
(f) producing a threat alert in response to any such selected differential
TOA below a predetermined value.
7. The method set forth in claim 6, including the further steps of:
identifying each said Other station from its reply messages;
determining the differential altitude of each identified Other station with
respect to Own station;
storing as entries said differential altitude data associatively with said
identity and differential time of arrival data, and
discarding all stored associated groups of identity and differential
altitude data with an altitude difference greater than a first
predetermined value above Own's altitude or a second predetermined value
below Own's altitude.
8. A collision avoidance system for detecting at an Own station the
proximity of transponder-equipped Other stations, comprising:
(a) means for receiving interrogation messages consisting of P1-P3 pulse
pairs transmitted by each SSR within operational range of said Own station
when said Own station is within the main beam of an SSR and also when said
Own station is within a side lobe of said main beam, and for also
receiving side lobe suppression (SLS) messages consisting of P1-P2 pulse
pairs or P2-only pulses associated with the main beam of each SSR when
said Own station is within operating range of said SLS transmission;
(b) means for receiving reply messages transmitted by any
transponder-equipped Other station in response to such main beam
interrogation messages during a predetermined period following reception
of each P3 pulse of an interrogation signal or P2 pulse of an SLS signal
at said Own station;
(c) means for determining, from the time relationships between each said
received P3 pulse or P2 pulse and each said received associated reply
message, differential time of arrival (TOA) data for each of said Other
stations, reply messages with respect to each said SSR;
(d) means for storing as entries in a running account each successive reply
message received by means (b) and the corresponding time of arrival data
determined by means (c) for a predetermined number of interrogation
repetition periods;
(e) means for matching from the entries stored in means (d) those messages
and corresponding TOA data that are substantially duplicated a
predetermined number of times;
(f) means for selecting in descending order of priority (1) reply messages
associated with P1-P3 interrogation signals; (2) in the absence of replies
associated with P1-P3 interrogation signals, reply messages associated
with P1-P2 side lobe suppression signals; and (3) in the absence of
replies associated with either P1-P3 interrogation signals or P1-P2 side
lobe suppression signals, reply messages associated with P2 only side lobe
suppression signals, and determining from the selected reply messages a
differential time of arrival (TOA) value for each reply message;
(g) means for comparing time of arrival data of reply messages selected in
accordance with step (f)(2) or step (f)(3) to measure short and long TOAs
that differ by 13 microseconds, and for subtracting 6 microseconds from
said short TOAs and 19 microseconds from said long TOAs to establish a
common TOA value;
(h) means for decoding said short TOAs as identity codes and said long TOAs
as altitude codes; and
(i) means for combining the outputs of comparing means (g) and decoding
means (h) to produce, in the absence of reply messages associated with
P1-P3 interrogation signals, identity and altitude data of Other stations,
and time of arrival (TOA) data for each said identified Other station,
equivalent to what the TOA value would have been had reply messages
associated with P1-P3 interrogation signals been received.
9. A collision avoidance system as set forth in claim 8, wherein said
system further comprises:
(j) means responsive to reply signals selected in accordance with step
(f)(1) for identifying each said Other station according to it reply
messages;
(k) means for determining from the time relationships between the P3 pulses
of each received interrogation signal and each said received reply message
elicited thereby differential time of arrival (TOA) values for each of
said identified Other stations with respect to each SSR;
(1) means for storing as entries in a running account each successive
identify obtained by means (j) and the associated time of arrival values
obtained by means (k) for a predetermined number of interrogation periods;
(m) means for matching from the entries to means (1) those identities and
time of arrival data that are substantially duplicated a predetermined
number of times;
(n) means for storing the entry for each such matched identity and
corresponding time of arrival data for a predetermined storage period;
(o) means for selecting the largest differential time of arrival related to
each said identified Other station;
(p) producing a threat alert in response to any such selected differential
TOA below a predetermined value; and;
(q) means for coupling identity and altitude data produced by means (i) to
said means (o) for selecting.
10. A collision avoidance system as set forth in claim 9, wherein said
system further comprises:
(r) means for determining the differential altitude of each said identified
Other station with respect to Own station; and
(s) means for coupling said differential altitude data to said means for
storing (n) for storage associatively with said identity and differential
TOA data.
11. In a collision avoidance system at an Own station which includes first
receiver means for receiving identity and interrogation messages composed
of differently spaced P1-P3 pulse pairs transmitted by the main beam of
each SSR within operational range of said Own station when said Own
station is within the main beam or main beam side lobes, of an SSR, and
for also receiving P2-only pulses or P1-P2 pulse pairs transmitted by the
side lobe suppression control (SLS) pattern associated with the main beam
of each SSR when said Own station is within operational range of said SLS
signals, second receiver means for receiving reply messages transmitted by
any transponder-equipped Other station in response to said interrogation
signals during a predetermined period following reception of said
interrogation signals by said first receiver and initiated by the P3
pulses of each said received interrogation signal, and means for producing
at Own station from said received interrogation signals and said received
reply messages from Other a differential time of arrival value for each of
said Other stations with respect to each said SSR, apparatus for
determining differential time of arrival values for each of said Other
stations in the event P1-P3 pulse pairs are not received, but P2 pulses
are received by said first receiver means, said apparatus comprising:
means for initiating said predetermined period by each received P2 pulse,
and
means for determining from the time relationship between each received P2
pulse and each received reply message elicited by an associated
interrogation signal, the identity and altitude of each said Other station
and differential time of arrival data for each of said identified Other
stations with respect to each said SSR.
12. Apparatus as set forth in claim 11, wherein said last-mentioned means
comprises:
means for storing as entries in respective running accounts the time of
arrival, relative to each received P2 pulse, of successive identity and
altitude reply messages representative of a predetermined number of
interrogation repetition periods,
means for selecting from said stored time of arrival values those TOA
values that differ by 13 microseconds and producing short and long TOAs,
means for subtracting 19 microseconds from said long TOAs and 6
microseconds from said short TOAs to establish a common TOA value
representative of what the TOA value would have been had P1-P3 pulse pairs
been received; and
means for decoding said long TOAs to obtain altitude codes and for decoding
said short TOAs to obtain identity codes.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to passive air traffic control and
collision warning systems which utilize interrogation signals transmitted
from a ground SSR station and associated transponder reply messages
transmitted by vehicles, such as aircraft, that are elicited by the SSR,
for determining the ranges, azimuth angles, altitude and identity of one
vehicle relative to another or to the ground station and, more
particularly, is concerned with improvements to systems of this type,
specifically the collision avoidance system described in applicants, U.S.
Pat. No. 4,486,755.
The system shown in U.S. Pat. No. 4,486,755, the disclosure of which is
hereby incorporated herein by reference, receives standardized
interrogation signals, having the waveforms shown in FIG. 1, transmitted
from a ground station at a frequency of 1030 MHz on a narrow rotating main
beam and in the side lobes of the main beam. The standardized
interrogation signal consists of three pulses each 0.8 .mu.sec. wide: a P1
pulse; a P2 pulse spaced 2.0 .mu.sec. from the P1 pulse; and a P3 pulse
spaced from P1 by either 8.0 .mu.sec or 21.0 .mu.sec. The P1 and P3 pulses
are transmitted by the main beam and also, unintentionally, by the main
beam side lobes which, unless suppressed, may be sufficiently strong to
interrogate nearby transponders, creating false replies. Referring to FIG.
2, the rotating directional antenna that has been employed over the past
several years, many of which likely will still be in service for many
years to come, produces a scanning beam 10 which is about 2.5.degree. to
3.0.degree. wide at its 3dB point and slightly wider at its suppression
control point.
In most SSRs, a second, static antenna omnidirectionally broadcasts a side
lobe suppression control pattern 12 containing P2-only pulses or P1-P2
pulse pairs, wherein the P2 pulse is synchronized with the P1 pulse in the
main beam, at a significantly higher level than the main beam side lobes,
the purpose of which is to prevent transponder replies to other than main
beam interrogation pulses when such main beam pulses exceed the P2 pulse
suppression signal level by a fixed amount. On some SSR control patterns
only P2 pulses are transmitted, which combine with the P1 pulses of
stronger main beam side lobes to create a P1-P2 suppression pair. More
specifically, the radiated amplitude of P2 at the transponder is (1) equal
to or greater than the signal amplitude of P1 from the greatest side lobe
transmission of the antenna radiating P1 (i.e., the rotating main beam 10)
and (2) at a level lower than 9dB below the radiated amplitude of P1
within the desired arc of interrogation. When main beam P1 pulse levels
exceed P2 pulse levels, P3 is no longer suppressed and P1-P3 pulse pairs
interrogate transponders that are in the main beam.
A P1-P3 pulse pair with a separation of 8.0 .mu.sec. between P1 and P3
transmitted on the main beam interrogates the identity (Mode A) of a
transponder-equipped aircraft, and a separation of 21.0 .mu.sec. between
P1 and P3 interrogates that aircraft's altitude (Mode C). A series of
about twenty such P1-P3 pulse pairs, one-half of which typically are Mode
A interrogations and the other half Mode C interrogations, is received at
a transponder, within a beam's width, during each 360 degree scan of the
rotating beam. During the period that the rotating beam is pointing at the
transponder, that is, the time the beam takes to scan approximately
4.degree., known as the "beam-dwell" time, the transponder replies in
accordance with the "un-suppressed" P1-P3 spacings of the interrogation
message. Interlaced Mode A and Mode C interrogation messages, such as
ACACAC, or AACAAC, are separated by intervals typically of about 2500
.mu.sec. but in the range between a minimum of about 2,000 .mu.sec. to a
maximum of approximately 5,000 .mu.sec. The broad SLS pattern, being
significantly stronger at all azimuths outside of the main beam skirt
(approximately 14-16 dB down from the main beam peak), prevents
interrogation pulse pairs from being received by a transponder unless they
are in the sector defined by the 3.degree.-4.degree. width of the main
beam.
Summarizing, a P1-P3 pulse pair transmitted on the SSR main beam will
interrogate an airborne transponder, causing it to transmit mode A and
mode C messages, only if the amplitude of the P1-P3 pulses received at the
transponder exceeds the amplitude of any received associated P2 pulses.
Each qualifying transponder within the 360.degree. scanning coverage of
the SSR main beam transmits in response a reply message on a 1090 MHz
radio frequency carrier back to the SSR, with a known delay, so that the
reply message is propagated along the path of the main beam and thus its
signal strength is increased by beam gain, and received by a 1090 MHz
receiver at the SSR. Each such 1090 MHz transponder transmission, known as
a "reply message" and depicted in FIG. 3, includes a pair of framing
pulses F1 and F2 separated by 20.3 .mu.sec. which define the start and
stop, respectively, of the message, between which thirteen information
pulses (twelve of which are currently used) are spaced in increments of
1.45 .mu.sec. from the first framing pulse, each of which is 0.45 .mu.sec.
wide and may or may not be present depending upon the content of the
message transmitted in reply to the 1030 MHz interrogation signal. The
format of the message contained between framing pulses F1 and F2 is
similar for any one of 4,096 identity codes transmitted. The absence or
presence of each of twelve information pulses establishes which code is
transmitted on 1090 MHz in response to the reception of an interrogating
P1-P3 pulse pair spaced by 8.0 .mu.sec.
Similarly, the format of the message contained between the framing pulses
is the same for any one of the altitude codes, which do not use D.sub.1
pulses, each of which represents the altitude of the aircraft to within
.+-.50 feet in 100-foot increments up to a maximum in excess of 125,000
feet. Thus, the structure of the reply message allows for the possibility
of 4,096 different code groups, each representing one or more pieces of
information such as identity or altitude of the responding aircraft. As
previously mentioned, 1030 MHz P1 and P3 interrogation pulses separated by
8.0 .mu.sec. when decoded elicit a reply code group transmitted on 1090
MHz representing identity. Similarly, a P1-P3 spacing of 21.0 .mu.sec.
elicits a reply code representing the altitude of a given aircraft. As
assigned by ATC or other authorities such as the military, the identity
code is set in by the pilot with a cockpit "digit switch", while the
altitude code is automatically established by a barometric altimeter and
an associated encoder. The identity code designations consist of four
digits, each of which lies between 0 and 7, inclusive, and is determined
by the sum of the pulse subscripts given in FIG. 3. The identity code of
the aircraft may be 1543, for example, which is represented by the
presence of A.sub.1 ; (B.sub.1 B.sub.4); C4; and (D.sub.1 D.sub.2) pulses.
The transponder automatically continuously transmits this identity code in
response to every received Mode A interrogation regardless of which radar
is interrogating, the beam width of the interrogating radar, or whether it
is a civil, military or European radar.
In a similar manner, in response to interrogation P1-P3 pulses spaced by
21.0 .mu.sec., the transponder automatically looks at an automatic
altitude encoder coupled to the aircraft's own barometric altimeter, which
automatically changes the code with changes in altitude according to a
pattern prescribed by the U.S. NATIONAL STANDARD FOR THE IFF MARK X (SIF)
AIR TRAFFIC CONTROL SYSTEM (Oct. 10, 1968), and the reply message
transmitted by the transponder is changed accordingly. Although the
altitude information is presented in the same pulse format as the identity
information, the ground system readily discriminates between Mode A and
Mode C replies to its interrogation, because the relatively long interval
between PRPs, and thus between interrogation messages, is such that only
during a specific period of, say 3,000 .mu.sec., representing a round trip
of about 250 nautical miles (3000/12 .mu.sec. per NM), following an
interrogation message wherein the P3 pulse is spaced from P1 by 8.0
.mu.sec., all aircraft that are within the beam and within 250 NM respond
with identity codes. Since the ranges of most SSR radars are limited to
about 200 miles line-of-sight, all targets reply within typically 2500 to
3000 .mu.sec. During the following PRP, during which, say a Mode C
interrogation is transmitted by the SSR, all aircraft out to a similar
predetermined range that are intercepted by the main scanning beam will
reply only with altitude codes. In this way there is no confusion between
identity and altitude replies even though both use identical signal
formats, because each pulse has a different significance. These identity
and altitude codes are interpreted by an airborne collision warning system
in the same way as does the ground station so as to provide collision
warning data on all nearby transponders.
In the passive threat warning and collision avoidance system described in
the '755 patent, an Own station receives interrogations from at least one
and usually multiple SSR's within operating range, not only when the main
SSR beam is pointing at it but also when Own station is illuminated by
lower level side lobes of one or more main beams, and capitalizes on time
of arrival (TOA) data from multiple SSRs to create a small cocoon of
airspace that represents the approximate range and near exact altitude of
any nearby transponder-equipped aircraft that may be a threat to Own's
aircraft. Use of such transponders is mandated in some 240,000 aircraft in
the United States alone and about 350,000 worldwide.
During a brief "listen-in" period of about 200 .mu.sec. initiated by Own's
reception of a P1-P3 decode, Own station receives replies transmitted by
transponders at Other stations in the general vicinity of Own station in
response to each interrogation from an SSR. The received replies are
decoded and using the P3 time of the associated interrogation message
received by the transponder's 1030 MHz receiver, produce time of arrival
(TOA) data for all surrounding aircraft and SSR stations within the
sensitivity range of the Own station's 1030 MHz and 1090 MHz receivers.
Operation of the '755 system depends on the amplitude of the side lobes of
the rotating main beam being sufficiently high that a P1-P3 pulse pair
would be received via the main beam side lobes so long as that receiver
was within a given operating range of an SSR. Thus, the '755 system
provides such TOA measurements not only during, but also before and after
passage of the main beam, so long as P1-P3 pulse pairs can be received;
the rotating main beam may be pointing in a direction other than at Own
station and interrogating other transponders. Consequently, it is
essential to the operation of the '755 system that it receive P1-P3 pulse
pairs, and the associated 1090 MHz responses, both before and after
passage of the SSR main beam through Own's station, throughout an angular
sector of about .+-.30.degree. straddling the main beam's axis. The
inability to receive P1-P3 pulse pairs in the deep nulls between the many
such side lobes limited the effectiveness of the system.
The last decade has witnessed an evolutionary change in the design of
ground SSR antennas, in particular the antenna system employed in SSR
systems of the type here under discussion. Several hundred U.S.-based
SSR's are now or are in the process of being equipped with an improved
antenna system which is electrically phased so as to create a narrow, main
scanning beam on which P1-P3 interrogation pulses are transmitted and
reply messages are received, and which has very low side lobes. The new
antennas usually do not include the static stand-alone antenna used in the
earlier system for omnidirectionally broadcasting a P1-P2 side lobe
suppression pattern, but, instead, employ antenna structure and radiating
elements integral and rotatable with the rotating main beam-forming
antenna structure for generating an SLS control pattern. As shown in FIG.
4, the SLS control pattern of this new system, containing either P1-P2
pulse pairs or only stand-alone P2 pulses, is generally "egg-shaped" in
the horizontal plane, or may have a narrow null along the main beam's
axis. The maximum signal of the SLS pattern, and therefore its maximum
range of reception, is aligned with the axis of the main beam 16 and
rotates with it; thus, the maximum signal level and therefore the range of
the rotating SLS pattern traces an imaginary circle 18 as it rotates with
the main beam. However, the signal strength is maximum only within a
sector approximately .+-.40.degree. wide which straddles the rotating main
beam. The signal level of the SLS control pattern in the direction of the
main beam typically is about 14 dB to 16 dB down from the peak amplitude
of the main beam and about 20 dB above the average level of the main beam
side lobes. The level of the control pattern above the side lobe level
varies with the angular displacement from the main beam, as much as 30 dB
at an angle of 180.degree. from the main beam, while averaging
approximately 20 dB above the main beam side lobes during a rotation
period. The new SLS pattern exhibits high signal levels, without deep
nulls, at all azimuths, within the .+-.40.degree. angular sector
straddling the main beam, outside of which there is some diminution in
level but still greatly exceeding the level of the main beams side lobes.
Unfortunately, however, this recent reduction in level of the main beam's
side lobes turns out to be a disadvantage to the '755 system, the
operation of which depends on reception of P1-P3 pulse pairs, not only
those contained in the main beam but also those transmitted in and between
adjacent side lobes. Consequently, the major reduction in side lobe level
provided by the improved SSR antenna significantly reduces the operational
range of the '755 system and, indirectly, the accuracy of its collision
warnings, by reducing the probability of receiving multiple SSR's at most
locations. As the population of improved antenna systems becomes larger,
the useful collision warning range of '755 systems could be reduced.
Adding to the challenge is the fact that of the approximately 3,000 SSR's
currently in service throughout the world, some already are using the
improved antenna system, others are in the process of being updated, and
others may continue using the "old" system, without change, for many more
years. It is projected that there will be a "mix" of old and new antenna
systems for approximately ten to twenty years before the "old" antennas
are totally phased out.
Thus, there is a current and compelling need for a passive threat warning
and collision avoidance system that is adaptable to the radiation
characteristics of both the "old" and the "new" SSR antenna systems. The
system should also be operable in geographical areas where only the P2
pulse is transmitted in the SLS control pattern, as is the case of SSR's
in England and some other European countries. Some U.S. stations such as
ASR-9 SSRs may also transmit only P2 pulses on the SLS control pattern.
Accordingly, the primary object of the present invention is to provide an
"adaptive" collision avoidance system embodying the principles of the '755
system and capable of operation with any of the three types of ground
radar transmission systems described above.
Another object of the invention is to extend the useful range of such
system from an SSR thereby to increase the probability that SLS signals
from two or more SSRs interrogating nearby targets will be received by the
collision avoidance system and thereby significantly reduce false alarms
and provide more precise measurement of pseudo-range.
Additionally, the system must be passive (that is, it should not itself
transmit for the purpose of detecting a potentially colliding airplane),
thus avoiding interference on either the 1030 MHz channel or the 1090 MHz
channel of the standardized SSR system.
The system should also be relatively simple and inexpensive to manufacture
so as to be economically feasible for owners of light aircraft such as
those used in general aviation.
SUMMARY OF THE INVENTION
In accordance with the present invention, which will be described in
association with the collision avoidance system shown in U.S. Pat. No.
4,486,755, but the principles of which are also applicable to other PSSR
systems such as that disclosed in U.S. Pat. No. 4,115,771. In the event
P1-P3 pulse pairs are not available, the system is adapted to use either
P1-P2 pulse pairs or "stand-alone" P2 pulses, for timing the "listen-in"
period of an Own station. More specifically, in the event of
unavailability of P1-P3 pulse pairs, of which P3 is normally used for
initiation of time of arrival (TOA) measurements, the system automatically
selects, as a second choice, P1-P2 pulse pairs because of their pulse
width and unique separation by exactly 2.0 .mu.sec. and selects, as a
third choice, stand-alone P2 pulses, for timing the "listen-in" period.
The amplitude of the P2 pulse contained in the SLS radiation pattern of
both the "old" and the improved SSR antenna system is greater than the
level of the main beam side lobes over an angular range of at least about
.+-.40.degree. from the direction of the main beam axis, which insures its
reception, before and after passage of the main beam through a
transponder, at ranges much greater than the range at which side lobes of
the main beam and nulls between them can be reliably received. For
example, by quadrupling the reception range in the .+-.40.degree. sector,
which is readily possible, by using P2 signal strength versus the much
weaker side lobe P3 signal strength and deep nulls between side lobes the
useful air-to-air collision protection area surrounding an SSR is
increased by a factor of sixteen. This increase would also apply to any
adjacent SSR's within Own's operating range, thereby providing large
overlapping protection areas.
That P2 time (derived from either the P2 pulse of a P1-P2 pair or a
"stand-alone" P2 ) can be used for synchronizing TOA measurements is based
on the different spacings between P2 and P3 pulses in Mode A and the P2-P3
spacing of Mode C interrogation messages, and the fact that Mode A and
Mode C reply messages elicited by an interrogating main beam mimic most of
the interrogation messages and other characteristics of the beam. From
Own's examination of the 1090 MHz "mimic" patterns, it is possible to use
P2 time to measure TOA values of reply messages. More particularly, since
the spacing between P2 and P3 pulses is exactly 6.0 .mu.sec. (8 minus 2)
for Mode A (identity) and 19.0 .mu.sec. (21 minus 2) for Mode C
(altitude), if P2 pulse time, instead of P3 pulse time, is used to
synchronize the start of the "listen-in" period during which the TOA of a
Mode A or a Mode C reply message is measured, the TOA can be corrected, if
the interrogation mode is known, to a "synthetic" P3 time. However, the
time difference between Mode A reply relative to P2 time and a Mode C
reply relative to P2 time will always be exactly 13.0 .mu.sec. (19 minus
6).
This 13.0 .mu.sec. difference in TOA measurements is critical to the
system's ability to identify and separate Mode A and Mode C replies. The
typical 1090 MHz "beam burst" of about twenty reply messages received at
the Own station transponder during each scan of a scanning SSR beam past
Other contains an approximately equal mix of Mode A and Mode C reply
messages. Importantly, all Mode A replies will have shorter TOAs, relative
to P2, than similar TOAs of Mode C replies. Because the "mimic" of the
beam received on 1090 MHz at Own station will contain the interrogating
radar's PRP, and the spacing and interlace pattern of the Mode A and Mode
C interrogation messages, by initiating time of arrival (TOA) measurements
with a P2 pulse, two "families" of TOA's, both referenced to P2 time, are
created, one family of essentially equal TOAs for Mode A and another
family of essentially equal TOAs for Mode C. By virtue of the 13 .mu.sec.
time differential between the two families of TOAs, and the fact that the
reply messages are contained in the same "burst", they can be readily
identified and separated.
Other objects, features and advantages of the invention will become
apparent, and its construction and operation better understood, from the
following detailed description when read in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1, to which reference has already been made, are waveforms depicting
standardized Mode A and Mode C interrogation signals;
FIG. 2, also referred to previously, is a simplified diagram of a ground
SSR interrogation system and depicts the radiation patterns of "old"
antenna systems;
FIG. 3, previously referred to, is a diagram showing the standardized code
characteristics of reply messages;
FIG. 4, to which reference has previously been made, shows the integrated
radiation pattern of the "new" antenna systems coming into use in SSR
systems;
FIG. 5 is a rectangular plot of the interrogation beam pattern and control
beam pattern typical of the new antenna systems;
FIGS. 6A and 6B placed together side by side as shown in FIG. 7 is a block
diagram, partly functional, of a collision avoidance system embodying the
invention; and
FIG. 8 is a geometric diagram used in explaining the operation of the
system of FIGS. 6A-6B with two favorably located SSRs.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Briefly referring again to FIG. 4, newer ground SSR stations differ from
the one depicted in FIG. 2 in the significant respect that they have a
phased array antenna which creates the interrogation radiation pattern
shown in plan view in FIG. 4 comprising a narrow clockwise rotating main
beam 16 and a much wider "egg-shaped" side lobe suppression control
pattern 14 which is aligned with, and rotates along with, the narrow
scanning beam. The side lobes of the main beam of the new antennas are
very low in signal power, which greatly reduces the useful range of
systems such as that described in the '755 patent that depends for its
operation on direct reception of P1-P3 pulses contained in the multi-beam
side lobes. All SSR ground stations, whether equipped with the "old" or
the newer antenna system, transmit on the main scanning beam at a
frequency of 1030 MHz, the internationally standardized interrogation
signals shown in FIG. 1, consisting of three 0.80 .mu.sec. pulses;
equal-amplitude P1 and P3 pulses separated by a specified interval and a
lower amplitude P2 control pulse separated from the P1 pulse by 2.0
.mu.sec. The ATCRBS (SSR) system relies on pulse amplitude comparison
between pulses P1 and P2 as received by the transponder to prevent
response to side lobe interrogation, and the Standards therefore specify
that the radiated amplitude of P2 at the antenna of the transponder shall
be (1) equal to or greater than the radiated amplitude of P1 from the side
When main beam P1 pulse levels exceed P2 pulse levels, P3 pulses are no
longer suppressed and the transponder replies by transmitting on 1090 MHz
over the desired arc of main beam interrogation. The signal strengths at
different azimuths outside the main beam's angular sector are such that
the P2 or P1-P2 combination is always greater than the P1-P3 combination
and thereby "suppress" any transponder, preventing it from receiving P3
pulses contained in the side lobes.
Side lobe suppression control pulses P2, synchronously locked to the timing
of the main beam P1-P3 pulses, are radiated at the same frequency using
the same time-shared transmitter, i.e., 1030 MHz, on a control pattern. In
much of the United States the control pattern is as depicted in FIG. 5, a
predominant P2 pulse, a P1 pulse 3 dB down from the level of P2, both of
which dominate a P3 pulse, with the P1-P2 pair dominating the P1-P3 side
lobe pulse pairs. In Great Britain and in some European countries the
control signal of FIG. 5 consists of only the P2 pulse which predominates
any received P1 or P3 pulse outside the main beam and, accordingly, will
suppress the transponder when a P1 from a strong side lobe combines with
the control pattern's single P2.
The relative amplitude levels of the main beam 10 and its side lobes and
the SLS control pattern of the "old" antenna system shown in FIG. 2 are
quantitatively depicted in FIG. 5, wherein all levels are indicated in
terms of dB down from the peak level (0 dB) of the main beam 10 used for
transmitting interrogation signals on 1030 MHz. The peak signal level of
the control beam typically is 16 to 18 dB below the peak level of the main
beam. At any azimuth the relative amplitude of the pulses changes with the
rotating antenna's instantaneous direction such that the level of pulse P2
is greater than that of P1 in all directions except in the direction of
the main beam. Similarly, as shown in FIG. 4, the relative amplitude of
the pulses change with antenna direction such that the signal strength of
the P2 pulse is greater than that of any P1-P3 pair in all directions
except for the narrow angular sector of the main beam. Thus, the "old" and
the improved antenna systems both produce radiation patterns which contain
P2 pulses greater in strength than P1-P3 pulses over a wide angular sector
except in the main interrogate beam which essentially bisects the angular
sector. Importantly, this insures long range reception of P2 pulses, both
before and after passage of the main beam through a transponder's
location, whether radiated by an "old" or a "new" antenna, so as to be
available for TOA timing purposes in the event normally-used P3 pulses are
absent.
Referring now to FIGS. 6A and 6B, a receiver 20 is designed to receive via
an antenna 22 standard interrogation signals and side lobe suppression
signals transmitted at the SSR frequency of 1030 MHz from a ground
station. The antenna may be a highly directive antenna pointed at a
distant radar in the case of a passive ground radar, or it may be
omnidirectional for an airborne application; the invention will be
described in an airborne environment. Because of constraints placed on the
location of antennas on aircraft, it is typical to use a single antenna
that both receives at 1030 MHz and transmits at 1090 MHz located on the
bottom of the aircraft, especially in the case of small general aviation
aircraft, although sometimes both top and bottom mountings are used with
two receivers to create a diversity system. The bottom antenna mounting is
preferred in a low-cost system because the ground antennas are beneath the
aircraft with the consequence that a bottom mounted antenna receives a
stronger signal than an antenna mounted on the top of the aircraft.
The output of receiver 20 is applied to a threshold device 24 arranged to
pass to a pulse width discriminator 25 any output from receiver 20
exceeding a predetermined threshold level. The pulse width discriminator
eliminates all pulses except those having a pulse width which satisfies
the 0.80 .mu.sec. width and other specifications of standard interrogation
pulses. The qualifying pulses are passed to a P1-P3decoder 26 designed to
provide an output on line 26A when an identity (Mode A) interrogation is
received, or an output on line 26B when an altitude (Mode C) interrogation
is received. These outputs are applied as information inputs to a switch
28.
The P1-P3 decoder 26 also provides an output on line A which represents the
P3 pulse of each received and decoded interrogation containing P1 and P3
pulses spaced by either 8.0 .mu.sec. or 21.0 .mu.sec., establishing that
the receiver is being interrogated by the main beam of an SSR or that the
SSR is sufficiently close to Own that the signal strength of the main beam
side lobes is high enough for P1-P3 pulse pairs to be received
consistently. All P3 pulses provided on line A by decoding of received
P1-P3 pulse pairs are passed with first priority by a priority selector
device 32 for use as the synchronizing signal for measuring TOA's.
The 0.8 .mu.sec. pulses passed by pulse width discriminator 25 are applied
to a P1-P2 decode 27 which provides an output on line B when the received
interrogation passed by threshold device 24 and pulse width discriminator
25 contains two 0.80 .mu.sec. wide pulses spaced, between their leading
edges, by 2.0 .mu.sec., which establishes that the system is receiving
P1-P2 pulse pairs conveyed by the SLS wide beam control pattern of an SSR
within the extended operating range of such stations. The amplitude of P1
compared to P2 can differ, but a pulse pair decode can occur so long as
both are above threshold within a typically 50 db dynamic range. An output
on line B is assigned second priority by priority selector device 32 and
is coupled to a box 82 labeled "P1-P2" for utilization in a manner to be
explained later.
The interrogation pulses passed by threshold device 24 and pulse width
discriminator 25 are also applied to a pulse storage device 36 which is
repeatedly opened and closed by a reset line 38 at 50.0 .mu.sec. (.+-.25
.mu.sec.)intervals to provide an approximately 50 .mu.sec. "window" during
which the applied signal is examined to determine whether any other pulse
or pulses similar to a single P2 pulse have appeared either before or
after it. Thus, absent P1-P3 pairs, P1-P2 pairs, or "stray" pulses within
the 50 .mu.sec. window, a "stand-alone" P2 pulse can be detected. The
output of this "window", which will be either an 0.8 .mu.sec. wide pulse
or multiple similar pulses,, is applied in parallel to a processor which
includes a unit 40 that examines the stored data and passes only one
single 0.8 .mu.sec. wide pulse, if present, and to a unit 42 which
examines the same stored data and detects if there are other 0.8 .mu.sec.
wide pulses contained within the 50 .mu.sec. "window" period. If unit 42
determines that multiple pulses are not present, a "kill data" bus
incorporated therein is not activated and precludes an output from unit
42; when the "kill data" bus is not activated a single pulse detected by
unit 40 is passed to one input of a unit 44 labeled "Single P2" and
provides an output on line C, establishing that only a single P2 pulse
exists within the 50 .mu.sec. window. If, on the other hand, unit 42
detects additional pulses, the "kill data" from unit 40 applied to a
second input of gate 44 will kill or terminate such pulses including a
single P2 pulse detected by unit 40.
The just-described process is available again immediately after either the
"killing" of multiple 0.80 .mu.sec. pulses or the passing of a stand-alone
P2 pulse. Since, on average, 1600 P2 pulses can occur in a single rotation
of the main beam (4.0 seconds/rotation.times.400 (PRP)), the killing of a
few stand-alone P2 pulses that may result from mutual interference of
multiple SSRs is generally insignificant in low and medium densities of
SSR stations and the system will operate without them. For example, if the
P2 pulse is killed because of the presence of multiple pulses in the
window, which is wide enough to include P1-P3 messages, and this occurs
say four times during the beam rotation period of four seconds, sixteen
out of 1600 stand-alone P2 pulses would be lost, but the same sixteen
would not necessarily be lost during the next beam rotation. However, on
such occasions the P1-P3 pairs or P1-P2 pairs would be used to create the
"P 3 time" via priority selector 32. Because the beam rotations of
multiple adjacent SSRs and, accordingly, their interferences, are random,
and the system is flexible enough to utilize data from two or three beam
rotations, of two or three SSRs, the loss of such P2 pulses on an single
rotation is insignificant. In a multiple SSR environment the priority can
change continuously during a period corresponding to average beam rotation
times. The single P2 pulses produced by this process are coupled to a box
84 labeled "P2 Only" for utilization in a manner to be explained.
The P2 pulses derived from either box 82 or box 84 are applied as one input
to a switch 110, and P3 pulses, that is, the priority #1 output of
priority selector 32, are applied as a second input to the switch. Switch
110 is so designed that in the absence of P3 timing pulses from selector
32 it feeds P2 pulses via line 112 to a line 68. If, on the other hand,
priority #1 P3 timing is available from priority selector 32, switch 110
instead applies the P3 pulses via line 112 to line 68 for utilization in a
manner to be described.
Reviewing the operation of the SSR in the just-described context, and
assuming another example of a single SSR received at Own transmitting only
P2 pulses (stand-alone, absent P3 absent P1) on its SLS control pattern
and a main beam rotation period of five seconds, for about 99% of the time
after the main beam carrying P1-P3 pulses passes the receiver, P2 pulses
will be present at the PRP of the SSR, which, for example, might be 400
pulses per second, thereby producing about 2000 "stand-alone" P2 pulses
nearly all of which pass through the 50-.mu.sec. window. For the single
SSR example, during the relatively long period of about 4.8 million
.mu.sec. for each rotation of the main beam in an English or European
radar, the "stand-alone" P2 pulses will be separated by about 2500
.mu.sec., with the consequence that there will be a probability of only
about 50/2500, or 2% of possible "stray" P2 pulses being present in the 50
.mu.sec.-wide window and passed as a true P2 pulse to priority selector
32. Because P2-only pulses are somewhat less reliable than a true P1-P2
pair for synchronizing the start of "listen-in" time, they are assigned a
lower priority, priority #3, by priority selector 32. Should a maximum
main beam side lobe conveying P1 pulses combine with a P2 pulse, priority
selector 32 determine that P1-P2 priority be used. Thus, the priority
selector adapts to available signals continuously throughout the time it
takes for a main beam rotation.
The interrogation code priority selector 32 is so arranged that if adequate
P1-P3 data is available over a given period of time, for example a few
interrogation periods, then that data is used as the timing signal for
measuring TOAs, just as in the system described in U.S. Pat. No.
4,486,755. However, if the SSR beam is pointing elsewhere than toward Own
station, and its side lobes are at such a low level that adequate P1-P3
data is not available at Own's location, then, if employed in the United
States (with P1-P2 pairs on the control pattern), decoded P1-P2 data will
usually be available over a wide angular sector surrounding and moving
with the main scanning beam. The United States has a population of several
hundred SSR's that radiate precisely separated (2.0 .mu.sec) pairs of P1
and P2 pulses on the SLS control pattern. If adequate P1-P2 data should be
unavailable, as, for example, in systems employed in England or other
European countries (and occasionally some United States SSRs) wherein only
P2 pulses are transmitted on the SLS control pattern, the lower priority
single P2 pulse data is utilized.
Thus, if priority #1 data is available for a "listen-in" period of 200
.mu.sec., priority selector 32 insures that priority #2 and priority #3
data is not used. If priority #1 data is missing, but priority #2 data is
available, the latter is utilized for that "listen-in" period in
preference to priority #3 data. After each "listen-in" period the priority
selector 32 selects in descending order the best, most likely and most
useful received data at that instant. Recalling that the 200 .mu.sec.
"listen-in" period is only 10% or less of the average SSR PRP of say, 2500
.mu.sec., following the close of the listen-in period there remains a
period of about 2300 .mu.sec. within which other radar interrogations can
be interleaved. This creates many synchronizing timing signals for TOA
measurements from which the best are selected by priority selector 32,
thereby maximizing the operating range of the equipment while maintaining
a high degree of integrity. The selections are automatic, which maximizes
the data from an SSR throughout its rotation period and avoids loss of TOA
data when deep nulls between main beam side lobes are pointing at the Own
aircraft. For example, the present invention should extend the useful
range from an SSR to an Own's airborne collision warning system
constructed in accordance with U.S. Pat. No. 4,486,755 by as much as four
or five times, say, from 20 miles to as much as 80-100 miles from an SSR.
This can increase useful coverage area of the herein described system
relative to each SSR's location by as much as twenty-five times, thereby
enhancing the desired overlap of multiple SSRs.
To repeat, when P1-P3 pulse pairs are not available, P1-P2 pulse pairs, if
available, are preferred over P2 -only pulses. However, if P1-P3 pairs and
P1-P2 pairs are both absent, P-2 only pulses can, as will be explained
presently, be used to initiate a 200 .mu.sec. "listen-in" period for TOAs.
In addition to being "adaptive" to the "old" and "new" antenna systems,
the just-described decoding/priority selection process is "adaptive" in
the sense that it is able continuously to examine received 1030 MHz
signals to determine every few hundred microseconds (e.g. 200-300
.mu.sec.) which of the three possible priorities are available and which
of them should be utilized at that instant to time the start of the
"listen-in" period. If, for example, interrogations are received from two
SSRs, one of which has P1-P2 pulse pairs on the SLS control pattern and
the other has only P2 control pulses, and P1-P3 pulse pairs are received
from neither, the system can extract useful information from both,
concurrently, as they interrogate other aircraft in the vicinity of Own.
If too many SSRs are present, overload control 119 can limit SSR data to
say the closest four or five SSRs.
Information contained in reply messages transmitted by Other aircraft is
received at the Own station during each scan of the scanning beam of an
SSR located within operating range. A 1090 MHz receiver 60, provided with
an antenna 61 preferably mounted on the top of the aircraft and designed
to receive standard transponder reply signals, is connected to a reply
decoder 64 via a threshold device 62 designed to pass any output from
receiver 60 exceeding a given threshold level, which level may be
controlled by a sensitivity time control generator 66. STC generator 66 is
controlled by timing pulses on line 68, which may be P3 or P2 pulses
depending on the output of switch 110, to initially provide a relatively
high threshold level, and then reduce the level over a period of, say 5
.mu.sec., thereafter maintaining the lower level so as to receive weaker
replies until the next P3 or P2 pulse occurs. If the equipment is embodied
in a passive ground radar (PSSR), the receiver antenna 64 may be the
highly directive, switched beam antenna system described in applicant
Litchford's U.S. patent application Ser. No. 07/813,137, filed Dec. 23,
1991, pointed directly at the responding aircraft; if embodied in an
airborne system, antenna 61 must be omnidirectional and preferably is
mounted on top of the aircraft. Receiver 60 may be similar to usual
transponder receivers but receiver 20 is about 20 dB more sensitive so as
to be capable of operating at, typically, -91 dBm sensitivity.
A "listen-in" gate generator 70 is connected to line 68 and arranged to
produce a gate signal of about 200 .mu.sec. duration following each P3 or
P2 pulse applied to line 68. The gate signal on line 70a enables the reply
decoder 64, which in the absence of the gate signal is disabled. When
enabled, for about 200 .mu.sec., decoder 64 produces an output on lines 72
and 74 which represents either the identity or the altitude information
contained in the current reply message, As shown in FIG. 5, each message
contains an initial framing pulse F1 and a second framing pulse F2 which
follows F1 by 20.3 .mu.sec., the interval between them containing thirteen
sub-intervals, of which twelve are currently used, in each of which a
pulse may or may not be present, providing for possibility of 4096
different codes, each code representing one or more pieces of information.
Since all messages are elicited by interrogation messages alternating
between Mode A and Mode C in synchronism with P3 pulses carried by the
scanning SSR beam, and both Mode A and Mode C replies utilize the same
twelve sub-intervals for carrying information, it is essentially
impossible to determine at Own's receiving station, using P2 timing
without more data, whether a given message is a response to a Mode A or to
a Mode C interrogation.
For example, a "burst" of about twenty reply messages are received by
receiver 60 from an Other aircraft during each 360.degree. scan of such
Other aircraft by the scanning SSR beams. Each 1090 MHz burst, which
represents 50 to 100 milliseconds of main beam dwell time depending upon
the type of radar (i.e. whether it is a rapidly rotating airport radar or
a slowly rotating en route radar), will produce on line 74 for application
to a beam burst signal processor 76 a mix of pulse messages representing
identity and altitude of each of the aircraft surrounding the Own station
out to the maximum reception range of Own's 1090 MHz receiver (say, 30-40
miles). Processor 76 is enabled by a P2 pulse on line 86, the source of
which will be described presently. Each data burst on line 74 is a "mimic"
(or imitation) of the SSR beam (or beams) that are interrogating the
airspace surrounding Own's transponder and all nearby transponders. The
mimic characteristics include exact, unique SSR spacing of interrogation
messages (often known as the SSR/PRP characteristics). Since all SSRs are
on the same RF channel, they are effectively finger printed or identified
by each having a unique PRP "signature" for interrogating; the period may
be fixed or it may be staggered. The mimic also creates replicas of the
Mode A and Mode C main beam interrogation patterns, such as AACAAC,
ACACAC, etc., the exact beam rotation period for any SSR that is being
received, the exact measured value of TOAs for Mode A replies and the
exact value of TOA measured for Mode C replies when the P2 pulse is used
as the start of the listen-in window in place of the P3 pulse start time
used in the '755 system.
For the sake of simplicity it will be assumed that the main beam's width
interrogates any transponder within its range of coverage, out to 100
miles and throughout an azimuth of 360.degree., and will elicit twenty
replies: ten Mode A reply messages and ten Mode C reply messages.
Utilizing the above-discussed observation that because of exact
standardized spacing of the P1, P2 and P3 pulses, the measured TOA at Own
of Other's identity (Mode A) reply messages, using P2 time, is exactly and
always 13 .mu.sec. shorter than the measured TOA of reply messages from
the same Other transponder station elicited by the main beam's Mode C
interrogation. When activated by a P2 pulse on line 86 (indicating a lack
of P3 data), beam burst signal processor 76, using techniques extensively
described in the literature, organizes the "burst" into two "histograms",
a "short TOA" histogram of identity code pulses and a "long TOA" histogram
of altitude code pulses. The "short" and "long" TOA histograms are applied
to units 78 and 80, respectively, which correlate the F1-F2 pulses of a
respective histogram relative to P2 time to determine the time of arrival
(TOA). Because in this example there are ten short TOAs, by creating a
histogram the processor 76 shows that the ten agree with each other
closely enough (in TOA and code content) to belong to the same "family"
and therefore to a single Other transponder station, and thus amenable to
autocorrelation. Similarly, the ten "long TOAs" within the time duration
of the beam burst are in sufficiently close agreement to belong to the
same "family" and also capable of being autocorrelated. A typical 1090 MHz
burst of twenty reply messages, spaced 2500 .mu.sec. apart, has a duration
of 20.times.2500 .mu.sec.=50 milliseconds. The following a P2 pulse any
associated 20.3 .mu.sec. message will be received.
The "short" and "long" TOAs are individually correlated, using P2 pulses
derived from either priority #2 data (box 82) or priority #3 data (box
84), and applied via line 86 to both of correlators 78 and 80. While each
of the TOAs may be of any specific length, depending upon Other station's
location relative to that of Own's station, because they are referenced to
the P2 pulse and processor 76 is activated by the P2 pulse, the "short"
and "long" TOA values in the burst of messages always must differ in
length from each other by 13 .mu.sec. Although a TOA value may be anywhere
in the range from 0.1 .mu.sec. to 200 .mu.sec., for purposes of the
discussion to follow, the "short TOA" will be arbitrarily assumed to be 46
.mu.sec. and the "long TOA" will then, of necessity, be 59 .mu.sec. These
values are indicated in blocks 88 and 90, and a comparator 92, shown
connected between these blocks establishes that the TOA value represented
by block 88 is shorter by 13 .mu.sec. than the TOA value represented by
block 90. It will be understood that this 13 .mu.sec. difference would
also be satisfied if, for example, the "short" TOA were 51 .mu.sec. and
the "long" TOA were 64 .mu.sec.
The correlated "short" and "long" TOAs which may, for example, contain a
message composed of A.sub.1, B.sub.2, C.sub.4 and D.sub.1 pulses
distributed between framing pulses F.sub.1 and F.sub.2, are applied to
similar correlators 94 and 96, respectively, which autocorrelate the code
information reply pulse-by-reply pulse. Since 4096 different Other's
identity "messages" are possible, when for example two or more successive
code patterns agree exactly, the probability of an erroneous output from
code correlators 94 and 96 would be less than one in sixteen million
(i.e., (4.times.10.sup.3).times.(4.times.10.sup.3). Thus, the system
provides the same enormous discrimination between "false" and "true" codes
as that available in current SSR systems.
A "true" TOA of 40 .mu.sec. for the "short" TOAs is obtained by subtracting
(depicted by block 98) 6 .mu.sec. (i.e., the P2-P3 spacing in Other's Mode
A) from the 46 .mu.sec. TOA represented by the autocorrelated code output
of correlator 94, the same as if P1-P3 pulse pairs had been received and
the TOA timed with respect to the P3 pulse instead of P2. Thus, the
described correlations automatically identify the family of "short" TOAs
as Mode A identification codes, which is outputted from block 100.
It is important to recognize that the TOA and the code structure really can
never be separated; since the TOA is referenced to the F1-F2 framing
pulses, specifically, F2 timing relative to the P3 timing (P3 actual or
P2-corrected time), the information contained between F1 and F2 is always
"locked-in". The code information may occasionally be garbled, but it is
nevertheless locked to TOA and cannot be separated. One or two garbled
messages, if present in a burst, are ignored as they do not correlate with
the several others.
A "true" value of 40 .mu.sec. for the "long" TOAs is obtained by
subtracting 19 .mu.sec. (Other's P2-P3 Mode C interrogation spacing) from
the 59 .mu.sec. TOA represented by the autocorrelated output of code
correlator 96, as depicted by block 102. Because of the above-described
separation of families of "long" TOAs (representing Mode C) from families
of "short" TOAs (representing Mode A), the "true" TOAs depicted by blocks
100 and 104 are both 40 .mu.sec. Other's ten Mode A replies on 1090 MHz
agree exactly with Other's ten Mode C replies by the described arithmetic
corrections. Consequently, twenty consistent TOA measurements are
available at Own from reply messages received from Other which can be
separated to provide an altitude code and an identity code of Other each
associated with the same TOA, as depicted by blocks 100A and 104A. This
enables the described diverse data to be combined, as represented by block
106, timed to "P2-time" but corrected after the beam burst to P3 time,
just as if it were timed from two P1-P3 decodes of 8.0 .mu.sec. and 21.0
.mu.sec. Arbitrarily assuming a Mode A code of 1253, and a Mode C code
representing an altitude of 3000 feet, by the above process it is
determined from twenty replies from this aircraft (i.e., No. 1253) that
the true TOA is 40 .mu.sec. and true altitude is 3000 feet.
Thus, the just-described system produces: an output on line 105
representing altitude information contained in the current reply message;
an output on line 107 representing identity information contained in the
current reply message; and an output on line 109 representing the distance
between Own and Other. The output on line 105 is applied to an altitude
comparator 111 having as a second input data provided by an encoding
altimeter 113 representing Own's altitude encoded in a similar format.
Comparator 111 produces an output representing the difference between
Own's and Other's altitudes when a Mode C reply occurs. The output of
comparator 111 is an information input to switch circuit 28.
Briefly reviewing the priority selection process, if priority #1 P3 timing
is available from priority selector 32, it is fed via line 114 to a gate
generator 116, the function of which will be described presently, and via
switch 110 and line 112 to line 68. In the absence of P3 pulses from
selector 32, P2 pulses produced at the output of either box 82 or 84 are
applied to line 86 and are also coupled via switch 110 and line 112 to
line 68. Line 68 is also connected to an overload control circuit 119
arranged to control the threshold level of device 24, as in a standard
ATCRBS transponder. It will be understood that any one or all three of the
signals outputted by priority selector 32 can be operating concurrently
depending upon the number and relative positions of multiple 1030 MHz SSRs
within the surrounding environment, and the strength of the signals that
are present. The extended ranges afforded by P2 timing increases the
likelihood that at least two SSRs will be received, thereby creating
throughout a much greater airspace higher accuracy and much fewer false
alarms than the pseudo-range methods described in U.S. Pat. No. 4,486,755.
The output of electronic switch 110, with P1-P3 pulse pairs always taking
precedence if available, can similarly be fed to appropriate connection
points of the PSSR system described in U.S. Pat. No. 4,115,771.
Returning now to the description of the collision avoidance system, a clock
generator and counter 118 is arranged to be reset by each P3 pulse
appearing on line 68, and to apply the current count, which may be a
numerical representation of the number of microseconds elapsed since the
last preceding P3 pulse was applied to counter 118. Each F2 pulse applied
to a gate 120 transfers the current count to line 122. The output of gate
120 on line 122 represents the differential time of arrival TOA of a
received interrogation and the corresponding received reply from a
transponder at an Other station. Clock generator and counter 118 is not
enabled by P2 pulses on line 68.
Whenever P3 is present on line 68, reply decoder 64 produces an output on
line 72 representing either the identity or the altitude information
contained in the current reply message. This output is applied to
comparator 111 and to switch circuit 28; comparator 111 produces an output
representing the difference between Own's and Other's altitude when a Mode
C reply occurs. The output of comparator 111 in response to a Mode A reply
will be spurious. In either case the output of comparator 111 is an
information input to switch 28.
Multiple line 72 is connected to supply all decoded outputs timed to P3,
both altitude and identity, from decoder 64 as information inputs to
switch circuit 28. When a P1-P3 identity interrogation is received, line
26a is energized to actuate switch 28 to pass the identity message to
switch output line 124. The output of comparator 111 at this time is
discarded. When a P1-P3 altitude interrogation message is received,
decoder 26 energizes line 26b, thereby activating switch circuit 28 to
apply the output of comparator 111 to line 124, discarding the input from
line 72.
Lines 122 and 124 are connected to a reply storage device 126, which may
comprise a plurality of digital registers arranged in known manner to
store associatively the TOA and identity or differential altitude
information corresponding to approximately twenty successive reply
messages. Preferably, the differential altitude information is stored
associatively with the identity and differential time of arrival data. The
information contained in each new reply message displaces the oldest such
stored information, so the storage device 126 maintains a running account
of identification and associated TOA and differential altitude
information.
A comparator 128, when enabled by P3 via gate generator 116, compares the
associated entries in storage device 126 with each other to select those
nearly identical entries that appear currently in the reply storage device
126. When such a match occurs the respective entry is transferred to a
selector device 130. The gate generator 116, which is similar to the
"listen-in" gate generator 70, is arranged to enable the comparator 128
for a period, beginning at the end of the listen-in gate, of sufficient
duration for completion of the operation of comparator 128.
The output of comparator 128 may and generally will, include several
entries containing the same identity information but substantially
different TOA information. The selector 130 rejects all such entries
except the one containing the largest TOA, which it transfers, together
with the associated identity and differential altitude information, to a
selected reply storage device 132. Storage device 132 is similar to device
126, but retains its entries for a period somewhat longer than the longest
radar beam rotation period to be expected, say fifteen seconds. If during
that time a new entry with a larger TOA value is presented, the new larger
value of TOA is substituted for the old, smaller value associated with
that particular identity.
It will have been recognized that the processing performed by reply storage
device 126 and comparator 128 is functionally equivalent to that
accomplished in the processing of "short" and "long" TOAs 78 and 80 when
beam burst signal processor 76 is enabled by the presence of a P2 pulse on
line 86. As just mentioned, when P1-P3 pulse pairs are available, reply
storage 126 device stores approximately twenty successive reply messages,
which is the equivalent of storing a total beam burst, just as is done by
the signal processor 76. In other words, when P2 is present on line 68,
signal processor 76 is activated and all information contained in about
twenty successive reply messages, which takes about 50 milliseconds
(50,000 .mu.sec.) to complete, is processed relative to P2 time. When P3
is present on line 68, signal processor 76 is disabled and the
approximately twenty successive reply messages are, instead, stored in
reply storage device 126 and compared in comparator to select those nearly
identical entries that appear currently in the reply storage device 126.
This being the case, whenever P2 pulses are being used for timing, the
identity and TOA information present on lines 107 and 109, respectively,
and the differential altitude information present on line 115, are all
applied to selector 130, instead of information supplied from comparator
128 when P3 timing is available. As before, selector 130 selects
information entries containing the largest TOA, which it transfers,
together with the associated identity and differential amplitude
information, to selected reply storage device 132.
The storage device 132 is connected to a threat detector 134 designed to
transfer, following a delay of 15 seconds, any entry containing a
differential altitude of less than a given valve, such as 3000 feet, and a
TOA of a given valve, such as less than 36 .mu.sec., to a display logic
device 136. At the same time, detector 134 provides an output on line 138
to start an alarm timer circuit 140 which may be similar to "listen-in"
gate generator 70, but designed to provide an output also lasting about 15
seconds. The output of timer 140 actuates an alarm device 142.
The display logic device 136 converts the output of detector 134 to a form
suitable for display on an identity indicator 144, a differential altitude
indicator 146 and a pseudo-range indicator 148. The pseudo-range
indication is a display of the differential TOA in terms of distance,
i.e., one-half the distance radiation travels during the time TOA. This is
what is meant by pseudo-range and corresponds to the actual range to a
degree that depends upon the positional relationship between Own and Other
stations and the SSR. The pseudo range is never greater than the actual
range. When Own and Other stations are both interrogated by a number of
SSR's, the likelihood of which is enhanced by the present invention, the
largest value of the pseudo-range associated with a particular Other may
closely approximate the actual range of said Other.
Referring to FIG. 8, which is a plan or map-like representation showing the
locations of an Own, an Other and two SSRs, line 201 represents the
distance from SSR-1 to Own, line 202 represents the distance from SSR-1 to
Other, and line 203 represents the range between Own and Other. The
differential time of arrival T1 in this case is the difference between the
sum of the travel times over paths 202 and 203 and the travel time over
path 201, generally expressed in microseconds. Any particular time T1
defines an ellipse such as 204, which is a locus of Other's position,
i.e., time T1 signifies only that Other is at some unspecified point on
ellipse 204.
It will be seen in FIG. 8 that lines 201 and 202 are approximately parallel
and thus T1 is very nearly twice the propagation delay along line 203, the
true range between Own and Other. Thus (cT1)2, referred to herein as the
pseudo range associated with SSR-1, is essentially equal to the true
range, where c is the propagation velocity.
Line 205 represents the distance from SSR-2 to Other. In this case the
differential time of arrival T2 defines ellipse 207 as a locus of Other's
position. Owing to the positional relationship between Own, Other and
SSR-2, the pseudo range associated with SSR-2, that is (cT2)2, cT2 is
considerably less than the true range, and may be shown to be a little
more than one-half the true range. Regardless of the relative position of
Own and any Other station and any SSR, the pseudo range can never be
greater than the true range and generally will be somewhat less.
Therefore, in a multiple SSR environment the largest determined pseudo
range to a particular Other is always selected by selector 130 as the
value most nearly equal to the true range. Thus, the larger TOA associated
with SSR-1 is selected as representative of "pseudo-range" of Other from
Own.
When an Other station is much closer to the SSR than Own station the pseudo
range may become a small fraction of the true range and, if the Other is
within the differential altitude limits, may initiate a threat detection
when in fact no threat exists. Such false threats are minimized by the
action of the STC generator 66 of FIG. 6A controlling the threshold device
62 to reject relatively weak replies received within a few microseconds
after reception of an interrogation.
While the principles of the invention have been described with the aid of a
block diagram of a currently preferred embodiment, it will now occur to
those skilled in the art that the invention can be modified in numerous
ways without departing from the spirit of the invention. It is the
intention, therefore, that the invention not be limited except as defined
by the appended claims.
Top